The present invention relates to a zoom lens system, and more particularly to a compact zoom lens system suitable for use in a 35 mm type twin lens camera.
It has been noted that the a zoom lens system is widely used in various types of cameras nowadays because it is capable of conveniently zooming the focal length of the lens system to facilitate the use of the camera. The zoom lens system having a variable zooming ratio, for example, from 1.3 to 2.0, falls into two categories. Typically, the first category consists of a front lens group having a negative refractive power and a rear lens group having a positive refractive power. An airspace is reserved between the front lens group and the rear lens group, and it is possible to effectuate variation of the focal length of the lens system by varying the airspace. Such a conventional zoom lens system is particularly suitable for use in the well-known single lens reflex cameras. However, such kind of zoom lens system has a serious disadvantage is that it has a long back focal length and a large diameter, and therefore making it impossible to make the zoom lens composition compact.
In view of the drawback of the first category of the prior art mentioned above, the second category of the zoom lens system is therefore designed to intend to shorten the back focal length and reduce the diameter of the lens system. Typically, the second category of the zoom lens system consists of a front lens group having positive refractive power and a rear lens group having negative refractive power, thus forming a so-called telephoto type zoom lens system. An airspace is also reserved between the front lens group and the rear lens group for zooming. This kind of zoom lens system is characterized in that it has a relatively shorter back focal length relative to the first category zoom lens system and therefore making it possible to be compact and sufficiently lightweight.
Various prior patents has been developed in this field. For example, U.S. Pat. No. 4,682,680 discloses a zoom lens system comprising five lens elements as a whole.
U.S. Pat. No. 4,838,669 discloses a zoom lens system having positive refractive power and a rear lens group having negative refractive power. The front lens group of the lens system consists of a positive meniscus lens, a negative lens, and at least one positive lens. The rear lens group of the lens system consists of a positive lens, a biconcave lens, and a negative meniscus lens.
U.S. Pat. No. 4,682,860 discloses a zoom lens system comprising a front lens groups having a positive refractive power and a rear lens group having a negative refractive power. The first lens group includes at least two positive lens elements and at least one negative lens element; the second lens group includes at least one positive lens element and at least one negative lens element. It is noted that the fourth embodiment as shown in FIGS. 9A and 9B of this U.S. patent comprises five lens elements forming a zooming lens system. However, it has lens elements with relatively steep radii of curvature and thus is relatively costly to manufacture.
U.S. Pat. No. 4,991,945 disclosed a zoom lens comprising five lens elements. The first three elements form a positive lens group and the last two elements form a negative lens group.